At least 21 people were killed in the southern Philippines today in a massacre that has shocked a country long accustomed to election violence.

A convoy of about 40 people was hijacked by 100 gunmen at a checkpoint in Maguindanao province, according to the military. Thirteen women and eight men were found dead three miles away, and the army and police were searching for other hostages. Some of the victims were beheaded, their bodies mutilated, local officials said.

"Never in the history of journalism have the news media suffered such a heavy loss of life in one day," Reporters Without Borders said in a statement.

"The frenzied violence of thugs working for corrupt politicians has resulted in an incomprehensible bloodshed."

The group included the wife and two sisters of Ismael Mangudadatu, the deputy mayor of Buluan township. They were going to the nearby Shariff Aguak township to file his nomination papers for the position of governor of Maguindanao province, running against Datu Andal Ampatuan, the head of a powerful family.

Mangudadatu said his wife and relatives were among the dead. He said his wife had called him by phone shortly before she and her entourage were abducted. "She said ... they were stopped by 100 uniformed armed men ... then her line got cut off," he said.

He accused the Ampatuans of carrying out the killings. Ampatuan has been elected governor of Maguindanao three times, always unopposed. Most of the 22 mayors in his province are sons, grandsons or relatives. Two of his sons have been killed in violence linked to clan wars.

In a 2007 interview with Reuters, Ampatuan said people did not run against him or his family members because they had little chance of winning.

"It's because of popular support," he said. "Because I am so loved by the constituencies of the municipalities, they ask me to have my sons as representatives."

Maguindanao is part of the autonomous region in Mindanao created as part of a 1996 peace deal with a large rebel group, the Moro National Liberation Front.

A presidential adviser, Jesus Dureza, said today's massacre was "unequalled in recent history. There must be a total stop to this senseless violence." He said a state of emergency should be imposed in the area and all gunmen disarmed.

The president, Gloria Macapagal Arroyo, ordered her senior security officials to "personally oversee military action" against those behind the killings. "No effort will be spared to bring justice to the victims and hold the perpetrators accountable to the full limit of the law," she said in a statement.

Arroyo has called Ampatuan a valuable ally in the past. In the 2004 presidential elections, she won most of the votes in Maguindanao. In one town, her rivals did win a single vote.

Philippine elections are often marred by violence in the south as heavily armed groups fight for self-rule and political warlords maintain private armies. The decades-long Islamist insurgency has killed about 120,000 people.

Elections scheduled for May are for a president, vice-president, nearly 300 MPs in the two houses of Congress and more than 17,000 local officials. Gilberto "Gibo" Teodoro, a former defence secretary, who Arroyo has picked as her successor, is trailing in opinion polls behind two opposition senators, Benigno "Noynoy" Aquino and Manuel Villar. Even former president Joseph Estrada, who stood down amid a welter of corruption charges, has better ratings than the administration than Teodoro.